Abstract The presence or absence of metastasis is the most important determinant of prognosis and management of cancer. Spread of circulating tumor cells (CTC) via the peripheral blood is shown to be prognostic indicator of later overt metastases. Currently we have a very limited overall fundamental biological understanding of CTCs, and of the clinical significance of CTCs in individual patients' disease progression and treatment response. Several approaches have been described for capture or isolation of CTC from the blood, most have limitations and a very few allow culture of the CTC upon isolation. We are thus forced to study a finite, small number of CTC captured at static time points to yield limited cellular and molecular data, precluding our ability to study CTC dynamically for their functional aspects. We have developed a unique precisely engineered microfilter platform that effectively separates larger CTC from smaller blood cells, and have modified it for viable CTC capture. In parallel, we have also developed a novel method for tumor cell culture, which we refer to as conditionally reprogrammed cell (CRC) system, using a ROCK-inhibitor-treated feeder layer. We propose to integrate the two novel platforms to achieve viable CTC capture and culture, following two Specific Aims: 1) To combine the microfilter and CRC technologies to capture and reliably grow cancer cells from blood in a model system, and 2) To use combined technologies to capture and grow CTC from patients with metastatic prostate, breast, colon and lung cancer. We have substantial preliminary data which shows that our capture technology can efficiently capture viable tumor cells from blood in a syngeneic mouse model which can be cultured such that the cultured cells preserve the original phenotype. In a key advance, we have also demonstrated the ability to efficiently isolate viable CTCs and successfully establish cultures that can be available for downstream functional and molecular characterization. Our CRC technology can efficiently and reliably grow tumor cells, starting even from small numbers of founder cells, and that the CRC show morphologic, phenotypic and genetic fidelity to the tumor of origin. Our data also shows that the CRC established from clinical samples show functional utility through their use to predict response to drugs in actual patients. Thus, the combination of the microfilter and CRC systems proposed here for capture and culture of rare CTC has a high likelihood of success, and can transform the way we evaluate and manage patients with cancer. We also believe that successful CTC cultures will lead to an important and necessary insight into metastatic process that the present cancer model systems cannot provide.